Made the various is_* operations return booleans. This was discussed

Made the various is_* operations return booleans.  This was discussed
with Cawlishaw by mail, and he basically confirmed that to these is_*
operations, there's no need to return Decimal(0) and Decimal(1) if
the language supports the False and True booleans.

Also added a few tests for the these functions in extra.decTest, since
they are mostly untested (apart from the doctests).

Thanks Mark Dickinson